Prior to delivery of paper and pulp wastewater to a dissolved air flotation (DAF) unit, e.g., a flotation Krofta, the wastewater is pre-treated with chemical additives which aid in the retention and separation of cellulose fiber suspension, fillers and other dispersed particles from the water.
In the dissolved air flotation process, clarification is achieved by forming micron-sized air bubbles in the water-fiber suspension which attach themselves to the suspended fiber or ash and float to the surface where they can be skimmed off with a mechanical scoop. The air bubbles are formed by dissolving air under 60-90 psi pressure. When released to the atmosphere in the DAF unit, the gas comes out of solution producing bubbles which average 20 microns in size.
Another advantage of dissolved air is that the lifting action of the bubbles tends to concentrate solids at the surface often making it possible to recover solids at concentrations of 2-4%. DAF units are typically designed such that the aerated mixture is laid in the unit at essentially zero velocity. In circular units this is accomplished by matching the speed of rotation of the inlet manifold to the flow. This minimizes turbulence and cross flow allowing the unit to take full advantage of coagulation, flocculation, and the lifting action of the bubbles.
Despite the inherent efficiency of DAF units and recent improvements and innovations in design, in most cases it is desirable and cost effective to enhance their performance by using chemical additives. Such additives can increase throughput and aid in the removal of fillers such as clay, titanium, and calcium carbonate which are often in a highly dispersed state due to the charge balance of the influent.
Canadian Patent No. 1,004,782 discloses the use of a phenol formaldehyde resin in combination with a high molecular weight polyethylene oxide to improve the retention at the dewatering of cellulose fiber suspensions. It was determined therein that the polyethylene oxide facilitates agglomeration of the flocculations formed with the phenol formaldehyde resin whereby retention and clarification are facilitated.
Swedish Patent Publication No. 454,507 (assigned to Berol Kemi Ab) discloses that the retention and/or purification of cellulose fiber suspensions and clarification of wastewater within the paper, pulp or board industry may be improved through pre-treatment with phenol formaldehyde resin and high molecular weight polyethylene oxide in combination with a cationic starch derivative or a cationic cellulose derivative.
Both of the aforementioned conventional pre-treatment methods utilize a dry particulate polyethylene oxide flocculant to facilitate retention and clarification. That is, these conventional methods call for the addition of polyethylene oxide to wastewater by diluting dry particulate polyethylene oxide with water to approximately 0.2% by weight immediately before addition.
To complicate matters more, conventional chemical pre-treatment of bleached chemi-thermal mechanical pulp (BCTMP) and chemi-thermal mechanical pulp (CTMP) has proved to be both difficult and expensive. BCTMP and CTMP wastewater pose effluent challenges for a number of reason, i.e., (1) they possess an extremely high cationic demand, (2) they are high in colloidal fines and suspended solids, (3) they can exhibit poor settleability characteristics, (4) they can be extremely high in soluble colour bodies, (5) they normally have effluent temperatures in excess of 30.degree. C., (6) they readily generate an excess foaming tendency, and (7) the levels of BOD and COD are normally very high.
These factors can be found in almost all BCTMP wastewater streams. Each plant may have different methods of solids removal, i.e., settling clarifiers, dissolved air flotation, etc. Of particular importance in the treatment of BCTMP/CTMP wastewater are the following four factors.
During the refining stages, fines are released and removed to meet `Freeness` targets. These extremely small particles possess a high negative charge density.
Poor settleability can hinder clarifier performance. BCTMP mills that utilize peroxide for their bleaching and where peroxide levels in the effluent are high, i.e., 200-600 ppm, steps must be taken to ensure the peroxide has decomposed fully prior to the effluent entering the clarifier. This can be accomplished by the use of sodium sulfite, organic matter (biological sludge), or acidic reduction. The latter has yet to be discussed but essentially peroxide is very unstable at low pH's around 4.0. The decomposition of peroxide in an acidic environment is twice that in an alkaline environment. This is another reason BCTMP mills use of sodium hydroxide is very high, i.e., to create a stable environment for the peroxide bleaching step.
Mechanical pulping practices are such that an extremely high amount of colour bodies, lignins are released during the impregnation or chip softening stages. Here caustic soda and steam are often used to soften the chips prior to refining. The colour bodies are released during this stage and normally in excessive amounts.
Because BCTMP effluents are alkaline in nature, this causes the effluent to be subject to foaming. This tendency cannot be fully eliminated with defoamers as the solids contamination is extremely high.
One conventional system used for pre-treating BCTMP wastewater is commonly referred to as the trawl-method. This method is applicable both for process water and wastewater clarification. The function of flocculation totally differs from the function of a conventional water clarification system. This method involves the addition of a phenol formaldehyde resin to the wastewater. The resin adheres onto fines, thereby creating anchoring places for the polymer. Thereafter, a solution of dry polyethylene oxide is added to the treated wastewater wherein the PEO binds to the sites covered with the resin. A network is formed consisting of fines and polymer. This network entraps other suspended particles.
Use of the aforementioned phenol formaldehyde resin/dry polyethylene oxide program has a number of disadvantages: (1) expensive; (2) ineffective in treating some wastewaters; and (3) the phenol formaldehyde resin is extremely toxic. Moreover, the resin forms colloidal particles at a pH below 9. The particle size depends not only upon the pH but also the soluble materials in the process water. Normally, the smaller the particle size, the higher the activity of the resin. The phenol formaldehyde resin will typically lose its effectiveness when the particle size becomes too large.
Another method for pre-treating BCTMP wastewater involves the principal of charge neutralization. This means that large amounts of decharging chemicals have to be added in order to flocculate large amounts of highly charged suspended material. For example, charge neutralization is accomplished by the addition of a preflocculant such as a metal salt, making the suspended particles attract one another forming microflocs. Thereafter, an anionic polyacrylamide is added to form bridges between the microflocs resulting in larger flocs.
The pre-treatment program of the present invention is much more cost effective than the conventional phenol formaldehyde resin/dry PEO program. It is also more flexible and covers a wider range of waste compositions, not satisfactorily treatable with the resin/dry PEO program. The present inventor has discovered through extensive experimentation that cationic, water soluble coagulants demonstrate more effectiveness in satisfying the cationic charge demand of the process than conventional phenol formaldehyde resins. These cationic coagulants also help to flocculate the fine suspended substances.
In situations where it is not cost effective to add low molecular weight cationic coagulants (i.e., coagulants having a molecular weight less than about 1,000,000) due to the large cationic demand of the wastewater, the present inventor has discovered that high molecular weight, low cationic charge coagulants (i.e., coagulants having a molecular weight in the range between about 9,000,000 to about 15,000,000) can readily be substituted therefor. Such high molecular weight coagulants are less affected by the anionic trash in the pulp and paper process.
The present invention also provides many additional advantages which shall become apparent as described below.